Nearline-SAS: Who Dares Wins

Maybe you think NL-SAS is old news and it’s already swept SATA aside?

Well if you check out the specs on FAS, Isilon, 3PAR, or VMAX, or even the monolithic VSP, you will see that they all list SATA drives, not NL-SAS on their spec sheets.

Of the serious contenders, it seems that only VNX, Ibrix, IBM SONAS, IBM XIV Gen3 and IBM Storwize V7000 have made the move to NL-SAS so far.

First we had PATA (Parallel ATA) and then SATA drives, and then for a while we had FATA drives (Fibre Channel attached ATA) or what EMC at one point confusingly  marketed as “low-cost Fibre Channel”. These were ATA drive mechanics, with SCSI command sets handled by a FC front-end on the drive.

Now we have drives that are being referred to as Capacity-Optimized SAS, or Nearline SAS (NL-SAS) both of which terms once again have the potential to be confusing. NL-SAS is a similar concept to FATA – mechanically an ATA drive (head, media, rotational speed) – but with a SAS interface (rather than a FC bridge) to handle the SCSI command set.

When SCSI made the jump from parallel to serial the designers took the opportunity to build in compatibility with SATA via a SATA tunneling protocol, so SAS controllers can support both SAS and SATA drives.

The reason we use ATA drive mechanics is that they have higher capacity and a lower price. So what are some of the advantages of using NL-SAS drives, over using traditional SATA drives?

  1. SCSI offers more sophisticated command queuing (which leads directly to reduced head movement) although ATA command queuing enhancements have closed the gap considerably in recent years.
  2. SCSI also offers better error handling and reporting.
  3. One of the things I learned the hard way when working with Engenio disk systems is that bridge technology to go from FC to SATA can introduce latency, and as it turns out, so does the translation required from a SAS controller to a SATA drive. Doing SCSI directly to a NL-SAS drive reduces controller latency, reduces load on the controller and also simplifies debugging.
  4. Overall performance can be anything from slightly better to more than double, depending on the workload.

And with only a small price premium over traditional SATA, it seems pretty clear to me that NL-SAS will soon come to dominate and SATA will be phased out over time.

NL-SAS drives also offer the option of T10 PI (SCSI Protection Information) which adds 8 bytes of data integrity field to each 512b disk block. The 8 bytes is split into three chunks allowing for cyclic redundancy check, application tagging (e.g.RAID information), and reference tagging to make sure the data blocks arrive in the right order. I expect 2012 to be a big year for PI deployment.

I’m assured that the photograph below is of a SAS engineer – maybe he’s testing the effectiveness of the PI extensions on the disk drive in his pocket?


9 Responses

  1. […] of you might also consider that XIV Gen3′s use of Infiniband interconnect and NL-SAS drives brings new relevance to my two recent blog posts on those subjects : […]


  2. NetApp/IBM N-series and HDS also use SAS in their portfolios.

    The entire storage world is in transition from FC/SATA to SAS/NL-SAS. Some vendors are ahead of others, but it is largely dependent on their hardware line refresh cycles.

    The bigger questions is when SSD/EFD will be priced the same or lower than SAS. When that occurs, we’ll likely be looking at a SSD and NL-SAS world with SAS itself slowly going away.


  3. I think the short answer to that is “not within the next 5 years” check out the graph on my post from October last year


  4. Thank you for the informatoin in your post. It was exactly what I was looking for in relation to the differences of SAS & NL drives.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

%d bloggers like this: